The present invention relates to sensing circuits and in particular, to sensing circuits for sensing a voltage having a wide dynamic voltage range from negative voltage to a positive high voltage and which is able to detect a threshold crossing accurately and rapidly.
Requirements for this type of circuit are that the input signal range is wide and must be capable of receiving the wide input signal range without damage to the circuit. For example, a signal range of minus 10 volts to 200 volts is within the area of application of the present invention. Further, the input stage must have a very small delay, the threshold detection must be very accurate and the input must be electrostatic discharge (ESD) protected.
FIG. 1 shows one prior art circuit for sensing input voltages. A voltage divider comprising resistors R1 and R2 is clamped by a zener diode D1 provided at the integrated circuit input pin to a fast comparator located within an IC. A disadvantage of this circuit is that it is slow because of the external RC time constant provided by the resistor R1 and parasitic capacitance. Furthermore, it dissipates power within the voltage divider when the input is at a high voltage. Additionally, the circuit is expensive and requires space for the external components outside the IC.
FIG. 2 shows another prior art circuit employing a resistor R1 and a zemer diode D1. An advantage of this circuit is that it can be completely integrated in the IC. Disadvantages include the RC delay is still significant due to the tradeoff between power dissipated in the resistor R1 and the RC time constant, as a high value of R1 is necessary to achieve a low power dissipation. Further, this circuit requires a high voltage resistor R1 which is often not readily available.